The transient receptor potential vanilloid 1 (TRPV1) is a member of ion channels mainly localized on primary afferent neurons. Activation of TRPV1 on sensory neurons by chemical stimulants including capsaicin and resiniferatoxin, as well as low pH (<6), heat (>42° C.), and nucleosides such as ATP, leads to an influx of Ca2+ and Na+ ions through the channel, causing depolarization of the cell and transmission of painful stimuli. Unlike traditional analgesic drugs that either suppress inflammation (e.g. NSAIDs and COX-2 inhibitors) or block pain transmission (e.g. opiates), TRPV1 channel inhibitors aim to prevent pain by blocking a receptor where pain is generated. In patients, the expression of TRPV1 is up-regulated in a number of painful inflammatory disorders. TRPV1 as a pain target has been validated by genetic deletion and pharmacological inhibition experiments. The pungency of capsaicin and many other agonists at the vanilloid receptor clearly defines TRPV1 as a key transducer in the pain pathway. Characterization of TRPV1 mice, which lack both copies of the TRPV1 gene, shows a complete absence of thermal hyperalgesia associated with inflammation demonstrating the key role of TRPV1 in disease and providing impetus to develop selective TRPV1 antagonists as a novel pain therapy with the potential for an improved side effect profile compared to existing therapies. Many novel selective and chemically distinct TRPV1 antagonists have been identified and a number of these have been assessed in preclinical models of pain. Some of them reverse mechanical hyperalgesia in the Freund's complete adjuvant model of inflammatory pain in rodents. Others show efficacy in neuropathic pain models, in post-operative pain and in cancer pain. These data provide robust validation of this approach for the treatment of a broad range of pain conditions in humans.
In the bladder, the presence of TRPV1 was demonstrated in various cell types, including urothelium, detrusor muscle and fibroblasts. There is good evidence that TRPV1 in urothelium is functional. Capsaicin evokes an inward current similar to that seen in DRG neurons in patch-clamped human urothelial cells. Furthermore capsaicin induces calcium uptake in human urothelial cells culture which is blocked by the TRPV1 antagonists implying that the regulation of TRPV1 is similar in sensory neurons and urothelial cells. Overactive bladder (OAB) is a syndrome characterised by urgency (with or without urge incontinence), usually with frequency and nocturia, in the absence of other pathologic or metabolic conditions that might explain the symptoms. Differently from antimuscarinic compounds dominating the market of OAB that only act on the efferent components, TRPV1 antagonists, acting on sensory nerves or on urothelium, are effective in diverse experimental models of cystitis/overactive bladder without interfere with the physiological volume-induced avoiding contractions (VIVC) and distention of the urinary bladder in healthy animals.
The documented ability of citric acid as well as pungent compounds such as capsaicin to induce cough when delivered to the lungs of experimental animals and humans, combined with the contribution of TRPV1-sensitive nerves to airway hyper responsiveness and bronco constriction has led to a large degree of interest in the potential for targeting TRPV1 for the treatment of a range of respiratory diseases. These effects are thought to derive from the key contribution of TRPV1 which is highly expressed by sensory neurons and vagal afferents that innervate the airways, to the cough reflex. Preclinical studies have now demonstrated antitussive efficacy of a range of TRPV1 antagonists in rodent models.
Dry Eye is a chronic dysfunction on tear and ocular surface epithelium.
Changes in corneal osmolarity is a trigger key event in cytokine production and ocular inflammation which are main causes of Dry Eye.
There are evidences that TRPV1 signal induces pro-inflammatory cytokine secretion in the corneal epithelial cells and hyper osmolarity-induced cytokine production is prevented by TRPV1 antagonists in corneal epithelial cells.
These data provide a strong rational for the systemic and topical use of TRPV1 antagonists in the treatment of Dry Eye.